E.K.
Go to Lakeshore learning and get an art folder so that he can store his art in that. Tell him that he has to chose what art he wants to keep and that he can only keep it in the folder if the folder gets too full he has to chose what to throw away to put new things in. Give him one drawer to store his other papers (perhaps in a night stand) or a special bag. Same rule here. He only gets to put papers in that one place once it is full he needs to through something away to make room for new stuff. He probably just needs guidance on how to sort through his stuff. Every day after school have him empty out his backpack, go through his materials with him. Help him sort through what is important to keep and what is garbage.